Fix TigerXML reader. Incremental treetransforms CLI

- TigerXML: properly convert normal vs. secondary edges
- TigerXML: handle case of single terminal directly under root node
- add parsed_sents_iter() method to corpus readers, which reads corpus
  and yields trees one at a time without storing them.
- use parsed_sents_iter() to make the treetransforms CLI incremental;
  i.e., trees are read, transformed, and written one at a time.